In the modern processing of dairy products, pasteurization is relied upon to provide safe and high quality products for consumers. Pasteurization consists of a heat treatment step that destroys harmful microorganisms while minimizing the development of off-flavors. In order to achieve these objectives, the proper combinations of pasteurization temperatures and holding times are used.
Originally, a batch process was relied upon to pasteurize dairy products, but this method has largely been replaced by continuous processes, which are more efficient and result in higher quality products. Several continuous pasteurization processes are available. For example, in the High-Temperature, Short-Time (HTST) pasteurization of raw milk, each particle of milk must be heated to 72.degree. C. (161.degree. F.) and held at this temperature for 15 seconds. In Higher-Heat, Shorter-Time (HHST) pasteurization, milk is heated to various higher temperatures and held at correspondingly shorter times than in HTST pasteurization. Ultra-High Temperature (UHT) pasteurization is used to achieve sterilization but at some sacrifice in taste of the treated product.
Pasteurization conditions in the United States and elsewhere are specified by regulations. In the United States, the Grade "A" Pasteurized Milk Ordinance specifies that the holding tube in an apparatus for continuous pasteurization shall be designed to provide for holding every particle of milk or milk product for at least the time required in the definition of pasteurization. Furthermore, the holding tube shall be so designed that the temperature difference between the hottest and coldest milk in any cross section of the flow will not be greater than 0.5.degree. C. (1.degree. F.). The holding tube shall be installed so that no portion between the inlet and the temperature sensor is heated.
Milk Pasteurization Controls and Tests, 2nd edition, Food and Drug Administration, 1986, specifies other conditions. The required length of the holding tube for HTST pasteurization is determined by a salt conductivity test. For HHST pasteurization, however, the sensitivity of the instruments for the salt conductivity test is not sufficient to measure the residence time accurately. Therefore, the assumption is made that the dairy product moves through the holding tube in laminar flow wherein the maximum fluid velocity at the tube centerline is twice the average velocity. Thus, in order to assure proper treatment of every particle of milk, the holding time must equal twice the specified time given in the definition for pasteurization. This requirement provides for a large margin of safety since it has been shown that even the most viscous dairy product will not achieve laminar flow, and in the case of condensed skim milk, the maximum velocity is only 1.7 times greater than the average velocity.
3-A Sanitary Standards, published by International Association of Milk, Food and Environmental Sanitarians, provide that in HTST pasteurization the holding tube, including its inlet and outlet connections, be constructed of sanitary pipe and fittings. Said pipe shall have product contact surfaces with a polished finish, free of all imperfections such as pits, folds, and crevices.
An empty pipe, as specified in the regulations, makes a poor holding tube for continuous pasteurization processes. As already noted, the product at the center of the pipe travels at nearly twice the average product velocity. Furthermore, the product at the wall moves more slowly than the average velocity. The result is that whereas the processing of the fastest moving product is optimal, the slower moving product will be over-processed. Adjustments cannot be made to reduce the average holding time without incurring the danger of allowing some harmful microorganisms to escape in the treated product.
It is therefore an object of the present invention to provide a pasteurization process that will assure the complete destruction of harmful microorganisms and at the same time reduce the development of off-flavors and other undesirable properties.
A further object is to provide a process that is simple in design and economical to operate.
These and other objects, features, and advantages of the invention will be apparent from the accompanying drawing, FIG. 1, and from the following description.